This invention relates to dichroic mirrors which can, if the incident light has a spectral distribution of a continuous character, produce light energy of uniform distribution or free from ripples over the transmitted light components.
Dichroic mirrors have found their uses, for example, in the three-color component beam splitting system disposed in the path of a single incoming beam of light between the objective lens and the pick up tube of a color television camera. The incoming beam of white light impinging on the dichroic mirror is divided into two light components, one of which is reflected and another of which which is transmitted. The reflected and transmitted components are in complementary relationship to each other. The reflection and complementary transmission characteristics of the dichroic mirror are utilized in the aforesaid beam splitting system to provide color separation images of a given object being recorded in the pick up tubes. Most commonly, the necessary color separation is achieved by disposing two dichroic mirrors, namely, a blue-reflector dichroic mirror I and a red-reflector dichroic mirror II in a spatial relation such as shown in FIG. 1. Both mirrors transmit a green band while respectively reflecting the blue band or the red band, with the result that the beam of white light reflected from the object being recorded is separated, in passing through the beam splitting system, into red, green and blue light. It is desirable that the transmission characteristics of the blue-reflector and red-reflector dichroic mirrors are uniform in spectral distribution, or otherwise the distribution of the light energy in the green spectrum region cannot be made uniform so that high grade imagery is not effected without color shift.
One type of dichroic mirror as shown in FIG. 2 is formed having alternating layers of a material having a high index of refraction H and a material having a low index of refraction L vacuum deposited on a transparent substrate 3 in the order such that the first layer adjacent to the substrate being of high index material, and the outermost layer is exposed to ambient air. Vacuum deposition represents one method for forming the layers since it allows precise control of the thickness of the layers. To attain a maximum reflectance at a design wavelength .lambda. in air, the optical thicknesses of a high index layer and the next low index layer should be controlled in a ratio of either 3.lambda.4 : .lambda.4, or .lambda.4 : .lambda.4 in terms of the design wavelength .lambda. as far as blue-reflector and red-reflector dichroic mirrors are concerned. In the prior art, almost all the layers had an optical thickness of substantially an odd number of one-quarter wavelengths of the design wavelength. For this reason, conventional blue-reflector dichroic mirrors provide a transmission passband having a large ripple from 500 m.mu. to 600 m.mu. in a longer wavelength, range than the design wavelength as shown at curve a in FIG. 4 (for 3.lambda./4 : .lambda./4 : type multilayered structure) and at curve c in FIG. 5 (for .lambda./4 : .lambda./4 type multilayered structure), while conventional red-reflector dichroic mirrors provide a transmission passband having a large ripple from 500 m.mu. to 560 m.mu. in a shorter wavelength range than the design wavelength as shown at curves b and d in FIGS. 4 and 5.
As is the case in the dichroic mirror of the type shown in FIG. 2, another type of dichroic multilayer mirror as shown in FIG. 3 is formed of alternating layers of a material having a high index of refraction H and a material having a low index of refraction L. The thickness of the layers of FIG. 3 is in the same order as the thickness of the layers of FIG. 2. However, in FIG. 3, the layers are arranged in the reverse order, i.e. low index, high index, low index and so on, rather than high index, low index and high index as in FIG. 2. To maximize the reflectance at a design wavelength, therefore, the optical thicknesses of a low index layer and the next high index layer should be controlled in a ratio of either .lambda./4 : 3.lambda.4, or .lambda./4 : .lambda./4 in terms of the design wavelength .lambda., as far as prior art blue-reflector and red-reflector dichroic mirrors are concerned. With this arrangement, likewise as above, the blue-reflector dichroic mirror provides a transmission passband having a large ripple from 500 m.mu. to 600 m.mu. in a longer wavelength range than the design wavelength, while the red-reflector dichroic mirror provides a transmission passband having a large ripple from 500 m.mu. to 560 m.mu. in a shorter wavelength range than the design wavelength.